XXI BEDS OF INFUSORIA 525 



secondly, that the still fluid lava within was packed by the 

 centrifugal force, generated by the revolving of the bomb, against 

 the external cooled crust, and so produced the solid shell of 

 stone ; and lastly, that the centrifugal force, by relieving the 

 pressure in the more central parts of the bomb, allowed the 

 heated vapours to expand their cells, thus forming the coarsely 

 cellular mass of the centre. 



A hill, formed of the older series of volcanic rocks, and 

 which has been incorrectly considered as the crater of a volcano, 

 is remarkable from its broad, slightly hollowed, and circular 

 summit having been filled up with many successive layers of 

 ashes and fine scoriae. These saucer- shaped layers crop out on 

 the margin, forming perfect rings of many different colours, 

 giving to the summit a most fantastic appearance ; one of these 

 rings is white and broad, and resembles a course round which 

 horses have been exercised ; hence the hill has been called the 

 Devil's Riding School. I brought away specimens of one of 

 the tufaceous layers of a pinkish colour ; and it is a most 

 extraordinary fact that Professor Ehrenberg ^ finds it almost 

 wholly composed of matter which has been organised ; he detects 

 in it some siliceous -shielded, fresh -water infusoria, and no less 

 than twenty -five different kinds of the siliceous tissue of plants, 

 chiefly of grasses. From the absence of all carbonaceous matter. 

 Professor Ehrenberg believes that these organic bodies have 

 passed through the volcanic fire, and have been erupted in the 

 state in which we now see them. The appearance of the layers 

 induced me to believe that they had been deposited under water, 

 though from the extreme dryness of the climate I was forced 

 to imagine that torrents of rain had probably fallen during some 

 great eruption, and that thus a temporary lake had been formed, 

 into which the ashes fell. But it may now be suspected that 

 the lake was not a temporary one. Anyhow we may feel sure 

 that at some former epoch the climate and productions of 

 Ascension were very different from what they now are. 

 Where on the face of the earth can we find a spot on which 

 close investigation will not discover signs of that endless cycle 

 of change, to which this earth has been, is, and will be 

 subjected ? 



On leaving Ascension we sailed for Bahia, on the coast of 



* Monals. der Konig, A/cad. d. Wiss. zu Berlin. Vom April 1845. 



